A central issue of both developmental and cell biology is the control of epithelial growth and differentiation. The study of these processes has been hampered by the lack of suitable models which can maintain both growth and differentiation in vitro. There is increasing evidence to indicate that the matrix to which epithelial cells are attached may have important influences on cell behavior. We propose using the rat mammary gland as a model system to study the role that extracellular matrix components play in cell growth and differentiation. Previously described in vitro systems for culturing mammary cells on tissue culture dishes, or on in gels of stromal (type I & III) collagen are unable to adequately simulate the normal developmental sequence of growth and differentiation. In vivo mammary epithelial cells are anchored to a basement membrane composed of type IV collagen, lamin, glycosaminoglycan and other glycoproteins. We have previously shown that primary cultures of rat mammary epithelium preferentially attach to a type IV (basement membrane) collagen substratum compared to stromal collagen and require this matrix for normal growth. Despite this requirement, this collagen matrix does not induce differentiation or increase culture longevity. In an effort to more closely simulate the in vivo environment, we have developed a new system in which rat mammary cells are cultured on an extracellular matrix derived from whole mammary glands. Preliminary studies indicate that this matrix permits cellular differentiation to a greater extent than previously described systems, and promotes response to mitogenic hormones including estrogen. The matrix which appears to be organ specific, also greatly increases culture longevity. We propose to use this new system as well as substrata of defined basement membrane components to investigate their role in mammary growth and differentiation. We will determine if these substrata modulate the responses of mammary cells to mitogenic and lactogenic hormones. We will also study the regulation of basement membrane production and degradation. These studies may lead to improved methods for the culture of epithelial cells. In addition, they should lead to a broader understanding of the role of extracellular matrix components in epithelial growth and differentiation.